Energetics of (Super)Tidal Baroclinic Modes in a Realistically Forced Global Ocean Simulation

Abstract

In this study, we diagnose the spatial variability in the energetics of tidally generated diurnal, semidiurnal, and supertidal ($>2.5$ cycles per day) internal wave vertical modes (up to mode 6) in a 30-day forward global ocean model simulation with a 4-km grid spacing and 41 layers. The simulation is forced with realistic tides and atmospheric fields. Diurnal modes are resolved beyond mode 6, semidiurnal modes are resolved up to mode 4, and supertidal modes are resolved up to mode 2, in agreement with a canonical horizontal resolution criterion. The meridional trends in the kinetic to available potential energy ratios of these resolved modes agree with an internal wave consistency relation. The supertidal band is dominated by the higher harmonics of the diurnal and semidiurnal tides. Its higher harmonic energy projects on the internal wave dispersion curves in frequency-wavenumber spectra and is captured mostly by the terdiurnal and quarterdiurnal mode-1 waves. Terdiurnal modes are mostly generated in the west Pacific, where diurnal internal tides are strong. In contrast, quarterdiurnal modes occur at all longitudes near strong semidiurnal generation sites. The globally integrated energy in the supertidal band is about one order of magnitude smaller than the energy in the tidal band. The supertidal energy as a fraction of the tidal energy is elevated along semidiurnal internal wave beams in the tropics. We attribute this to near-resonant interactions between tidal modes of the same mode number.